In the field of lubricating oils, additives such as viscosity index improvers and pour point depressants have conventionally been added to lubricating base oils, including highly refined mineral oils, to improve the viscosity-temperature characteristics or low-temperature viscosity characteristics of the lubricating oils (see Patent documents 1-7, for example). Known methods for producing high-viscosity-index base oils include methods in which feed stock oils containing natural or synthetic normal paraffins are subjected to lubricating base oil refining by hydrocracking/hydroisomerization (see Patent documents 7-10, for example).
The viscosity index is commonly evaluated as the viscosity-temperature characteristic of lubricating base oils and lubricating oils, while the properties evaluated for the low-temperature viscosity characteristics are generally the pour point, clouding point and freezing point. Methods are also known for evaluating the low-temperature viscosity characteristics for lubricating base oils according to their normal paraffin or isoparaffin contents.